Variants:

For readers who have only learned the C position notes, lay out all the flashcards. If they want to claim a space with a note they haven’t learned, they should say only whether it’s higher or lower than the notes they do know.

For more advanced readers, use cards with ledger lines or intervals.

For an extra twist, make it Gravity Connect Four, where all pieces automatically fall down to the lowest available flashcard in their column.

For a bigger extra twist, make it Reversi, where if you flank an opponent’s pieces, they are replaced with your pieces. In this version, the goal is not to get four in a row, but to complete the grid and then count up who has claimed the most flashcards.